1. Field of the Invention
The present invention relates to an overheat detecting circuit. In particular, the invention relates to an overheat detecting circuit for detecting a semiconductor substrate temperature higher than a predetermined temperature.
2. Description of Related Art
In recent years, a power LSI (Large Scale Integration) has been widely used for the purpose of generating high voltage or a large amount of current. The power LSI generates high output power and thus, involves a large self-heating amount. Hence, a semiconductor substrate temperature tends to increase. There is a fear that elements on the semiconductor substrate are broken due to the high semiconductor substrate temperature. To overcome this, the power LSI generally incorporates an overheat detecting circuit as a protective circuit to avoid abnormal temperature increase of the LSI to thereby protect the LSI from breakdown due to overheating. Japanese Unexamined Patent Application Publication No. 6-169222 discloses an example of such overheat detecting circuits.
FIG. 7 is a circuit diagram showing a conventional overheat detecting circuit 700. The overheat detecting circuit 700 of FIG. 7 has a current mirror circuit composed of a current determinative resistor R1 and transistors MN71 and MN72, diodes D1 to DS as overheat detecting elements for generating a voltage V1, resistors R2 and R3 for determining a comparative reference voltage V2, and a comparator for comparing the voltage V1 and the comparative reference voltage V2. The operating voltage of the comparator ranges from a power supply voltage VCC and a reference voltage REF2 (VCC-6 V), and an operating voltage of the other elements ranges from the power supply voltage VCC and a reference voltage REF1 (VCC-2.5 V).
The conventional overheat detecting circuit 700 supplies a current determined with the current determinative resistor R1 to the overheat detecting element through the current mirror circuit, and then the comparator compares the voltage V1 with the comparative reference voltage V2 to obtain an output OT_L.
The diodes of the overheat detecting element cause a voltage drop of 0.6 V per diode when the semiconductor substrate is at ordinary temperature (for example, 25° C.). Further, the diode has a temperature characteristic of −2 mV/° C. Here, if a resistance value ratio between the resistors R2 and R3 is 3:2, the comparative reference voltage V2 equals (VCC-1.5 V).
The voltage V1 is lower than the comparative reference voltage V2, so a high-level voltage (for example, power supply voltage VCC) is output as the output OT_L until the semiconductor substrate temperature reaches an overvoltage detection temperature. Further, if the semiconductor substrate temperature exceeds the overheat detection temperature, that is, 175° C., a voltage drop per diode is 0.3 V, the voltage V1 equals (VCC-1.5 V). The semiconductor substrate temperature is much higher, and the voltage V1 exceeds (VCC-1.5 V). To that end, provided that the overheat detection temperature is 175° C., when the semiconductor substrate temperature exceeds 175° C., the voltage V1 becomes higher than the comparative reference voltage V2, so a low-level voltage (for example, VCC-6 V) is output as the output OT_L.
Through the aforementioned operation, the conventional overheat detecting circuit 700 changes an output voltage to detect overheating when the semiconductor substrate temperature exceeds the overheat detection temperature.
However, the conventional overheat detecting circuit 700 involves a variation factor, and exhibits a variation of ±10% of the reference voltage REF1 (VCC-2.5 V), a variation of ±10 mV of an input offset of the comparator, and a variation of ±40% of a resistance value of the current determinative resistor, for example. These variations lead to a variation in overheat detection temperature. For example, if all the variation factors vary to the maximum, an offset value of the overheat detection temperature is about ±25° C.
In some cases, the power LSI is used in high-temperature ambience such as an engine room of an automobile. Under such use environment, a high detection accuracy may be required for detecting the overheat detection temperature. For example, the circuit may be required to normally operate at temperatures of up to 175° C., and even upon the abnormal heat generation, the temperature should be 200° C. or smaller in some cases. To satisfy such conditions, even if the overheat detection temperature is 187.5° C., an offset value of the overheat detection temperature should be suppressed to ±12.5° C. or smaller. The conventional overheat detecting circuit 700 has a problem in that the overheat detection temperature largely varies and cannot be used for detection.